The present invention relates to a process for preparing silica-alumina carriers and to hydrogenation catalysts prepared therewith. The present invention also relates to a process for the reduction of the aromatic hydrocarbons content in hydrocarbon streams.
Most of the liquid hydrocarbon products obtained by refining crude oil (distillates, gasolines, . . . ) contain levels of aromatic hydrocarbons which can be harmful for further use. Indeed, aromatics are known to reduce the cetane index of diesel fuels. Also, aromatics evaporating from hydrocarbons are more toxic than aliphatics. Finally, aromatics generate more smoke and more soot particles during combustion. Consequently, more and more environmental regulations are set up to limit the aromatics content in solvents or in transportation fuels.
The reduction of aromatics content in hydrocarbon streams can be carried out by catalytic hydrogenation on metal catalysts. However, most of the refining streams contain certain levels of sulphur and nitrogen compounds which are known as poison of these catalysts.
Group VIII transition metals such as Co or Ni supported on a carrier are very active catalysts, but in the presence of sulphur they are converted into inactive sulphides.
The use of Group VI-Group VIII bimetallic sulphides such as those of Nixe2x80x94W or Nixe2x80x94Mo can be found advantageous since their activity is less sensitive to sulphur. However, very high hydrogen pressures are required to observe significant aromatics conversion.
Noble metals such as Pt or Pd and alloys of noble metals deposited on a carrier are also known as active aromatics hydrogenation catalysts. The Applicants have already developed such hydrogenating catalysts as described in U.S. Reissue 26,883. However, these catalysts are based on low-alumina silica-alumina carriers. Further the shaping of such carrier requires a pelletising step. Said pelletizing step is slow, expensive and difficult to handle due to abrasion of the equipment; further by this method it is not possible to obtain catalyst particles with a diameter lower than 3 mm.
The need to use hydrocarbon streams, e.g. petroleum distillates, with lower and lower aromatics content is still requiring further improvements of hydrogenation catalysts.
It is an object of the present invention to provide for a silica-alumina carrier as well as a process for its preparation.
It is another object of the present invention to provide for an hydrogenation catalyst based on the silica-alumina carrier of the invention as well as a process for its preparation.
It is a further object of the present invention to provide for a hydrogenation process of hydrocarbon streams in the presence of the hydrogenation catalyst of the present invention.
According to the present invention, the silica-alumina carrier can be obtained by a process comprising the following steps:
(a) mixing an aluminum compound with a silicon compound to obtain a solution, said aluminum compound being chosen among aluminum alcoholate and aluminum carboxylate, and said silicon compound being chosen among silicon alcoholate and siloxane compounds;
(b) hydrolyzing the solution from step (a) under acidic conditions at a temperature comprised between 50xc2x0 C. and 150xc2x0 C.;
(c) cooling the mixture coming from step (b) in order to obtain a gel;
(d) processing the gel from step (c) to obtain a paste by eliminating the excess of volatile compounds such as acid and water;
(e) extruding the paste coming from step (d) under the form of extrudates;
(f) calcining the extrudates from step (e) at a temperature comprised between 300xc2x0 C. and 700xc2x0 C. for at least a few hours to remove the organic materials and moisture.
According to another embodiment of the present invention, the hydrogenation catalyst can be obtained by depositing one or more metals of Group VIII on the silica-alumina carrier of the invention.
The Applicants have unexpectedly found that it is possible to obtain a significant reduction of the aromatic hydrocarbon content of hydrocarbon streams when using the hydrogenation catalyst of the invention in a hydrogenation process.
The Applicants have now found that starting with an aluminum compound such as an aluminum alcoholate or an aluminum carboxylate and dissolving said aluminum compound into a silicon compound such as a silicon alcoholate or a siloxane compound to obtain a homogeneous solution, leads to beneficial results when the so-prepared support is used in the process of the invention.
The aluminum carboxylate is preferably a C1-C4 carboxylate. As example of aluminum carboxylate, one can cite aluminum acetate, hydroxyaluminum diacetate and aluminum acetylacetonate.
As an example of a siloxane compound, one can cite polyalkoxy siloxane (preferably with 1 to 4 C1-C4 alkoxy radicals per Si atom) such as e.g. polyethoxy siloxane having 1-9 Si atoms per molecule.
It has been found particularly suitable to use Al and Si alcoholates wherein each alcoholate group has from 1 to 4 carbon atoms. Generally, it is preferred to use Al isopropylate and Si(OEt)4 or Si(OMe)4.
The solution obtained at step (a) preferably comprises from 8 to 40% by weight, preferably from 9 to 25% by weight and more preferably from 10 to 15% by weight of aluminum oxide versus total oxide (i.e. aluminum oxide+silicon oxide).
The hydrolyzing step (b) is preferably performed under weak acidic conditions (pH comprised between 2.5 and 4), for instance by pouring the solution from step (a) into acetic acid aqueous solution of 0.05 to 0.5 mole/liter. The hydrolysis is performed under stirring at a temperature comprised between 50xc2x0 C. to 150xc2x0 C. and preferably between 70xc2x0 C. and 90xc2x0 C.
According to the present invention the processing step (d) for preparing the silica-alumina carrier preferably consists of a drying step which is usually performed at a temperature comprised between 20xc2x0 C. and 100xc2x0 C., preferably between 50xc2x0 C. and 80xc2x0 C.
According to the present invention, step (e) can be performed either in an extruder to form extrudates or according to conventional pelletizing process. Preferably, the paste obtained at step (d) is processed in an extruder to form extrudates. Organic extrusion aids such as glycerol or methyl cellulose can be added to the mixture. According to a preferred embodiment of the present invention, these extrudates have a size lower than 3 mm, more preferably lower than 2 mm; this represents a great advantage because such processing was not possible with the prior art methods for preparing a low-alumina silica-alumina carrier. Indeed, the small size of the extrudates enhances the transportation of the reactants through the catalyst particles, and increases the volumetric activity for a given reactor.
In accordance with a preferred embodiment of the present invention, before the calcination step (f), the extrudates obtained at step (e) hereabove are allowed to dry between room temperature and 150xc2x0 C.
The calcination temperature of step (f) is comprised between 300 and 700xc2x0 C., preferably between 500 and 600xc2x0 C.
According to another embodiment of the present invention the paste obtained at step (d) may also be blended with kaolin in an amount up to 10 parts for 100 parts silica/alumina; kaolin is added as an additional extrusion aid.
As indicated hereabove and according to another embodiment of the present invention, the hydrogenation catalyst can be obtained by depositing on the silica-alumina carrier of the invention one or more metals of Group VIII, preferably from 0.1 to 1.5 wt % in total.
The impregnation is preferably performed with a solution of one or more salts or complexes of noble metals of Group VIII. A salt or complex of platinum is preferred. More preferably the impregnation is performed by using an acidic complex of Pt, with the pH of the initial Pt complex solution being sufficiently low ( less than 3, if possible less than 2) to favour Pt deposition while obtaining a good Pt distribution, but not too acidic to avoid carrier dissolution ( greater than 0.5, if possible greater than 1). More preferably the impregnation is carried out by using an acidic H2PtCl6 solution in sufficient amount to have a Pt deposit of about from 0.1 to 1.5 wt % Pt, preferably from 0.3 to 1 wt % Pt.
In case of impregnation with a palladium salt either alone or in conjunction with the Pt salt, the use of PdCl2 is preferred under similar conditions. In that case the amount of Pd deposited is of about 0.1 to 1.5wt % Pd, preferably from 0.3 to 1% Pd. Conjunction of Pt and Pd can be preferred since their joint use leads to better resistance to sulfur poisoning; a weight ratio of Pt to Pd of from 0.1 to 10 is most preferred.
As indicated hereabove and according to another embodiment of the present invention, a reduction of the aromatic hydrocarbon content is obtained when using the hydrogenation catalyst of the invention in a hydrogenation process of hydrocarbon streams.
The hydrocarbon streams are usually liquid hydrocarbon products obtained by refining crude oil (e.g. distillates, gasolines, . . . ), said products containing certain levels of aromatics hydrocarbons. The hydrogenation process of the present invention is particularly suitable for petroleum refinery distillates having a boiling range comprised between 60xc2x0 C. and 350xc2x0 C.
The hydrocarbon streams usually contain as a matter of practice a minimum of 0.1 p.p.m. by weight of sulphur, and an aromatic content of about 1 to 99% by volume.
The catalysts of the invention are less sensitive to sulphur in the feed than Pt/alumina catalysts. While not wishing to be bound by a theory, it is believed that the sulphur level in the feed partly determines the activity level of the catalysts of the invention (as do other reaction conditions as the temperature, the pressure, the amount of hydrogen and the contact time). If necessary, the sulphur content may be lowered by hydrogenating the hydrocarbon stream according to processes well known in the refining of hydrocarbon streams, e.g. in the presence of a catalyst comprising by way of an example, cobalt and molybdenum oxides supported on alumina.
More particularly, the catalyst of the invention may thus be used in a continuous method for reducing the aromatic hydrocarbon content of a hydrocarbon stream having a sulphur content preferably not greater than 1000 p.p.m. by weight (more preferably lower than 500 p.p.m.) and boiling in the range of 60xc2x0 C. to 350xc2x0 C., and hydrogenating said hydrocarbon stream on the catalyst whose silica-alumina carrier contains at least 75% by weight of silica, preferably 85% by weight of silica.
Hydrogenation is desirably carried out under the following conditions:
Hydrogen to hydrocarbon ratio: 100 to 3,000 liters hydrogen at normal temperature and pressure (N.T.P.) per liter of liquid feed, preferably 150 to 2,500 liters hydrogen (in English units about 500-17,000 s.c.f., preferably 850-14,000 s.c.f. per barrel of liquid feed).
It is of great advantage to use as high a temperature as possible in order to increase the reaction rate. Temperature should, however, be kept under a certain limit according to the thermodynamic equilibrium. Use of temperatures higher than this limit indeed favors the dehydrogenation reaction rather than the hydrogenation reaction, as well as secondary reactions such as hydrocracking.
The process of the invention may be carried out continuously for very long periods of time.